1. Field of the Invention
The present invention relates to method and apparatus for seeking a track of an optical information carrier, and more particularly to method and apparatus for seeking a desired one of a plurality of tracks formed on an optical information carrier in an optical information handling apparatus such as an optical disk device.
2. Description of the Prior Art
An optical information handling apparatus which records and reproduces information at a high density utilizing a light is known in the art. In an apparatus of this type, an information carrier made of a photosensitive recording material is rotated and a laser beam having a fine spot such as 1 .mu.m in diameter is directed onto the information carrier to form recesses or holes or to change a reflection factor or magnetization direction so that information is recorded and reproduced. Examples of playback only products are optical video disks and digital audio disks, and an example of a record/playback product is an optical disk device used in an electronic file system. Signals to be recorded are video signals, audio signals and computer digital signal and the application field is expanding constantly.
On the optical information carrier used in the optical information handling apparatus, information is usually recorded on information tracks each comprising a series of recording pits. On a record/playback type information carriers, optically detectable guide tracks are formed to assure recording of signal at a small track pitch and at a high density and to assure accurate reproduction of the signal recorded at high density. Examples of the guide tracks are grooves formed on a base of the information carrier (pre-grooving) and guide tracks as well as track addresses and sector addresses written on the information carrier by a high power laser (pre-formatting).
Many of such information tracks and guide tracks are formed on the information carrier. For example, several tens of thousands of tracks are formed on the optical disk concentrically or spirally. The optical information handling apparatus must have a tracking mechanism for allowing an information writing or reading light spot to precisely follow the track and a seek mechanism for shifting the light spot to a desired track. An example of a prior art optical information handling apparatus equipped with such mechanisms is shown in FIG. 1.
In FIG. 1, an optical disk 21 which is an information carrier is rotated by a motor 22 around a spindle 23. A plurality of guide tracks . . . 24n-1, 24n, 24n+1, 24n+2, . . . , have been previously formed on the optical disk 21 as shown by A, and a light spot 26 is irradiated to the track 24n by a light head 25. The light beam of the light spot 26 is emitted from a semiconductor laser 27 in the light head and focused onto the optical disk 21 through a collimeter lens 28, a beam splitter 29 and an object lens 30. A drive current to the semiconductor laser 27 is modulated with recording information and the information is recorded on the guide track 24n by the light spot 26 in a form of record pits. A portion of the light reflected by the optical disk 21 is transmitted through the object lens 30, reflected by a beam splitter 29, transmitted through a condenser lens 31 and directed to a photo-detector 32. The photo-detector 32 has two split photosensing planes an a border line therebetween is oriented parallelly to a direction of a tangential line of the guide track 24n. The reflected light forms an image of the guide track 24n on the photosensing planes. If the light spot 26 deviates from the guide track 24n, a distribution of light intensity on the photosensing planes varies. Accordingly, by differentiating the outputs from the split photosensing planes by a differential amplifier 33, a tracking error signal is produced at a terminal 34. The object lens is moved in a direction B by a mechanical device (not shown) in accordance with the tracking error signal so that the light spot 26 always irradiates the guide track 24n. The tracking control is effected in this manner.
A method for shifting the light spot 26 to a desired record track, that is, a track seek method is now explained. In the track seeking, the optical head 25 is moved by a mechanism (not shown) radially of the optical disk (direction C) so that the light spot 26 scans across the guide tracks . . . 24n-1, 24n, 24n+1, 24n+2, . . . Each time the light spot traverses the guide track, a light intensity on the photosensing planes of the photo-detector 32 gradually decreases. Accordingly, the traverse of the guide track can be detected by utilizing the output at the terminal 34. By counting the number of traversed tracks, the guide tracks onto which the light spot was shifted is identified. Thus, when the target number of tracks to be traversed is given and the movement of the light spot is started, the detection signal indicating the traverse of the track by the light spot is counted, and when the count reaches the target number of tracks, the movement of the light spot is stopped. In this manner, the desired track can be sought at a high speed. Similar tracking and seeking are effected in reproducing the information.
However, as described above, a track pitch is very small in a high density information carrier. For example, the track pitch of the optical disk is 1-2 .mu.m and a small defect area or dust causes partial defects of several to several tens of tracks. If such partial defects exist, the detection signal indicating the traverse of the track is not produced at the defect area even if the light spot traverses it. Accordingly, exact seeking is not achieved.